Field of the Invention
The present invention relates generally to a force sensor and an apparatus having the sensor for measuring weight. More particularly, the present invention relates to a force sensor using a displacement amplification mechanism and an apparatus having the sensor for measuring weight, whereby the displacement amplification mechanism enhances detection sensitivity of the force sensor by amplifying a displacement of an elastic body having high strength, thereby exactly measuring weight.
Description of the Related Art
Generally, a displacement of an elastic body is measured by using a force sensing resistor method, a pressure measuring method, a strain gauge method, etc. The conventional methods are shown in the following Table 1 in accordance with a performance.
TABLE 1ForcesensingStrainPressureCapacitiveresistorgaugesensortypeThickness andthin filmdetermineddependingthin filmsizeby size ofon diameterstructureof tubeNecessity forno needneedno needno needvoltageamplifierResponse timeslowfastslowfastAccuracygreatlywithin 3%decreasewithin 5%decreaseRepeatability40%/hour———per hourDurabilitybadgoodPossible togoodreduce
The force sensing resistor is in the shape of a thin film and is flexible such that the force sensing resistor can be designed in a small size. However, the force sensing resistor has low capacity, low durability, and slow response time. Therefore, the force sensing resistor is used as a switch obtaining activation/deactivation information, rather than for obtaining large amounts of information.
The pressure sensor has linear values within a measurement range of the pressure sensor, and has an error rate slightly changing depending on the measurement range. However, response time of the pressure sensor is irregular depending on a position of pressure. In addition, the pressure can be converted into force but there is a limitation in obtaining an exact measurement value of the force.
The strain gauge is the most commonly used, and expresses weight as an electric signal by converting a strain rate of a structure deformed by the weight into the electric signal.
When the weight is applied to a structure, the structure is deformed, and the strain rate of the deformed structure should be transferred to the strain gauge without distortion. However, an adhesive attaching the strain gauge distorts the strain rate of the deformed structure, and the distorted strain rate is transferred to the strain gauge. The adhesive attaching the strain gauge is formed of a high molecular substance, and has an amorphous structure that is heterogeneous. Therefore, mechanical properties of the adhesive are uneven such that it is difficult to predict the mechanical properties.
In addition, in a process of producing an adhesive layer disposed between a structure and the strain gauge, it is difficult to produce the adhesive layer having a uniform thickness. Moreover, in a process of hardening the adhesive layer, it is difficult to ensure even mechanical properties of the adhesive layer. In an enlargement of the adhesive layer, a lot of bubbles are present in the adhesive layer, and the bubbles degrade the mechanical properties.
In the meantime, the most important resistant material of the strain gauge is made in the shape of a grid having thin lines. The cross sectional shape of the resistant material is uneven such that it is hard for the resistant material to perform tensile-compressive strain based on tensile-compressive strain of the structure. Consequently, the electric signal (output voltage according to applied voltage) changing depending on the resistant material is uneven.
The description of the related art described above describes the conventional methods and disadvantages thereof, and does not affect claims of the present invention described later or the scope of the present invention.
A force sensor calculates force values by measuring a displacement of the elastic body caused by force. Various methods are used to measure the displacement. Since a measurement range is defined according to the various methods, the elastic body is designed to make the displacement of the elastic body meet the measurement range.
When capacity of the force sensor decreases, strength of the elastic body is required to be reduced to exactly measure the displacement. In particular, in the case of an optical method or a capacitive method measuring large displacement in comparison with the strain gauge method, the elastic body is designed to have low strength such that the elastic body has a large displacement, thereby limiting minimum capacity of the force sensor.
In addition, when a high-capacity sensor is produced, the displacement of the elastic body is required to meet the measurement range such that the elastic body is designed to have low strength. Accordingly, it is difficult to apply the force sensor to a case in which high strength is required to secure high precision of a mechanism.